Semiconductor Device and Manufacturing Method Thereof

ABSTRACT

An object is to obtain a semiconductor device with improved characteristics by reducing contact resistance of a semiconductor film with electrodes or wirings, and improving coverage of the semiconductor film and the electrodes or wirings. The present invention relates to a semiconductor device including a gate electrode over a substrate, a gate insulating film over the gate electrode, a first source or drain electrode over the gate insulating film, an island-shaped semiconductor film over the first source or drain electrode, and a second source or drain electrode over the island-shaped semiconductor film and the first source or drain electrode. Further, the second source or drain electrode is in contact with the first source or drain electrode, and the island-shaped semiconductor film is sandwiched between the first source or drain electrode and the second source or drain electrode. Moreover, the present invention relates to a manufacturing method of the semiconductor device.

TECHNICAL FIELD

The present invention relates to a semiconductor device and amanufacturing method thereof.

BACKGROUND ART

In recent years, development of thin film transistors (TFT) that can beformed over flexible plastic substrates or paper substrates has beenactively pursued in order to realize sheet displays, sheet computers,and the like. Consequently, it is an important challenge to respond to ademand that a TFT be able to be manufactured at a temperature that isequal to or lower than an allowable temperature limit of a substrate.

However, manufacturing the TFT at a temperature that is equal to orlower than the allowable temperature limit of the substrate means thatheating at high temperature cannot be performed, and this causesproblems. For example, a problem occurs in which an on current andmobility which are TFT characteristics are degraded due to insufficientcontact between a wiring and a semiconductor film.

As an example of a pixel TFT provided for a display device, there is abottom gate TFT in which a gate electrode is positioned under asemiconductor layer and a gate insulating film. Further, among bottomgate TFTs, there is a top contact type in which a source electrode and adrain electrode are positioned over the semiconductor layer (see FIG.2A), as well as a bottom contact type in which the source electrode andthe drain electrode are positioned under the semiconductor layer (seeFIG. 2B) (refer to Patent Document 1: Japanese Published PatentApplication No. 2005-223048).

A top contact type bottom gate TFT shown in FIG. 2A includes a gateelectrode 1002 over a substrate 1001, a gate insulating film 1003 overthe gate electrode 1002, and a semiconductor film 1004 over the gateinsulating film 1003. Further, electrodes 1005 each functioning as asource electrode or drain electrode (hereinafter referred to as sourceor drain electrodes 1005. Note that in this specification, “source ordrain electrodes” mean that each of the electrodes may function as asource electrode or a drain electrode) are formed over the semiconductorfilm 1004. When voltage is applied to the gate electrode 1002, a sourceregion or a drain region is formed in each of regions of thesemiconductor film 1004 that are in contact with the source or drainelectrodes 1005, and a channel forming region is formed in thesemiconductor film 1004 above the gate electrode 1002 and between thesource region and the drain region.

A bottom contact type bottom gate TFT shown in FIG. 2B includes a gateelectrode 1012 over a substrate 1011, a gate insulating film 1013 overthe gate electrode 1012, and source or drain electrodes 1014 over thegate insulating film 1013. Further, the bottom contact type bottom gateTFT includes a semiconductor film 1015 over the gate insulating film1013 and the source or drain electrodes 1014. When voltage is applied tothe gate electrode 1012, a source region or a drain region is formed ineach of regions of the semiconductor film 1015 that are in contact withthe source or drain electrodes 1014, and a channel forming region isformed in the semiconductor film 1015 above the gate electrode 1012 andbetween the source region and the drain region.

In a top contact type bottom gate TFT (FIG. 2A) and in a bottom contacttype bottom gate TFT (FIG. 2B) in which the source electrode and thedrain electrode are positioned under the semiconductor layer, contactbetween the semiconductor film and electrodes or wirings is poor if aheat treatment is not sufficiently performed, and an on current andmobility are degraded. Therefore, a heat treatment at around 300° C. isusually performed. However, when a substrate is made of a material witha low allowable temperature limit instead of a material with a highallowable temperature limit, such as plastic or paper, the heattreatment cannot be performed at such a temperature.

Further, although it is necessary that film thicknesses of electrodes orwirings are thick in order to lower wiring resistance, in the bottomcontact type bottom gate TFT shown in FIG. 2B, there is concern thatcoverage of the semiconductor film and the source or drain electrodesbecomes poor. Consequently, degradation in TFT characteristics occurs.

DISCLOSURE OF INVENTION

An object of the present invention is to obtain a semiconductor devicewith improved characteristics by reducing contact resistance between asemiconductor film and electrodes or wirings and improving coverage ofthe semiconductor film and the electrodes or wirings.

In a semiconductor device of the present invention, in order to reducecontact resistance between a semiconductor film and electrodes orwirings, source or drain electrodes are formed over and under thesemiconductor film.

Further, in order to improve coverage of the semiconductor film and theelectrodes, thicknesses of first source or drain electrodes that areformed under the semiconductor film is to be thinner than that of secondsource or drain electrodes that are formed over the semiconductor film.

Note that in this specification, a semiconductor device refers todevices in general which function by utilizing a semiconductorcharacteristic, and semiconductor circuits, electro-optical devices andelectronic devices including a semiconductor layer are all semiconductordevices.

The present invention relates to a semiconductor device including a gateelectrode over a substrate, a gate insulating film over the gateelectrode, first source or drain electrodes over the gate insulatingfilm, an island-shaped semiconductor film over the first source or drainelectrodes, and second source or drain electrodes over the island-shapedsemiconductor film and the first source or drain electrodes. The secondsource or drain electrodes are in contact with the first source or drainelectrodes, and the island-shaped semiconductor film is sandwichedbetween the first source or drain electrodes and the second source ordrain electrodes.

The present invention relates to a semiconductor device including a gateelectrode over a substrate, a gate insulating film over the gateelectrode, first source or drain electrodes over the gate insulatingfilm, an island-shaped semiconductor film over the first source or drainelectrodes, an island-shaped impurity semiconductor film added with animpurity imparting one conductivity type over the island-shapedsemiconductor film, and second source or drain electrodes over theisland-shaped semiconductor film, the island-shaped impuritysemiconductor film and the first source or drain electrodes.

The second source or drain electrodes are in contact with the firstsource or drain electrodes, and the island-shaped semiconductor film andthe island-shaped impurity semiconductor film are sandwiched between thefirst source or drain electrodes and the second source or drainelectrodes.

The present invention relates to a manufacturing method of asemiconductor device in which a gate electrode is formed over asubstrate, a gate insulating film is formed over the gate electrode,first source or drain electrodes are formed over the gate insulatingfilm, an island-shaped semiconductor film is formed over the firstsource or drain electrodes, and second source or drain electrodes areformed over the first source or drain electrodes and the island-shapedsemiconductor film. The second source or drain electrodes are in contactwith the first source or drain electrodes, and the island-shapedsemiconductor film is sandwiched between the first source or drainelectrodes and the second source or drain electrodes.

In the present invention, at least one of the gate electrode, the gateinsulating film, the first source or drain electrodes, the island-shapedsemiconductor film, an island-shaped impurity semiconductor film addedwith an impurity imparting one conductivity type described below, andthe second source or drain electrodes is formed by an ink-jet method.

The present invention relates to a manufacturing method of asemiconductor device in which a first conductive film is formed over asubstrate, a gate electrode is formed using the first conductive film, agate insulating film is formed over the gate electrode, a secondconductive film is formed over the gate insulating film, first source ordrain electrodes are formed using the second conductive film, asemiconductor film is formed over the first source or drain electrodes,an island-shaped semiconductor film is formed using the semiconductorfilm, a third conductive film is formed over the first source or drainelectrodes and the island-shaped semiconductor film, and second sourceor drain electrodes are formed using the third conductive film. Thesecond source or drain electrodes are in contact with the first sourceor drain electrodes, and the island-shaped semiconductor film issandwiched between the first source or drain electrodes and the secondsource or drain electrodes.

The present invention relates to a manufacturing method of asemiconductor device in which a gate electrode is formed over asubstrate, a gate insulating film is formed over the gate electrode,first source or drain electrodes are formed over the gate insulatingfilm, an island-shaped semiconductor film is formed over the firstsource or drain electrodes, an island-shaped impurity semiconductor filmadded with an impurity imparting one conductivity type over theisland-shaped semiconductor film, and second source or drain electrodesare formed over the first source or drain electrodes, the island-shapedsemiconductor film and the island-shaped impurity semiconductor film.The second source or drain electrodes are in contact with the firstsource or drain electrodes, and the island-shaped semiconductor film andthe island-shaped impurity semiconductor film are sandwiched between thefirst source or drain electrodes and the second source or drainelectrodes.

In the present invention, the impurity imparting one conductivity typeis phosphorus or arsenic.

In the present invention, the impurity imparting one conductivity typeis boron.

A source region and a drain region of a semiconductor device of thepresent invention each have a structure of being sandwiched from aboveand below by source or drain electrodes; therefore, contact resistanceof the source or drain electrodes and a semiconductor film can bereduced. Also, since there are two layers of the source or drainelectrodes, wiring resistance can also be reduced. Further, since firstsource or drain electrodes that are formed under the semiconductor filmcan be formed to be thin, coverage of the semiconductor film can beimproved to improve TFT characteristics. Consequently, reliability of asemiconductor device of the present invention can also be improved.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 shows a cross-sectional view of a semiconductor device of thepresent invention;

FIGS. 2A and 2B each show a cross-sectional view of a conventionalsemiconductor device;

FIGS. 3A to 3D each show a manufacturing step of a semiconductor deviceof the present invention;

FIG. 4 shows a result of calculating transistor characteristics of a TFTof the present invention;

FIGS. 5A to 5E each show a manufacturing step of a semiconductor deviceof the present invention;

FIGS. 6A to 6D each show a manufacturing step of a semiconductor deviceof the present invention;

FIGS. 7A and 7B each show a manufacturing step of a liquid crystaldisplay device of the present invention;

FIGS. 8A and 8B each show a manufacturing step of a liquid crystaldisplay device of the present invention;

FIG. 9 shows a manufacturing step of a liquid crystal display device ofthe present invention;

FIG. 10 shows pixel portion of a liquid crystal display device of thepresent invention;

FIGS. 11A to 11D each show a manufacturing step of a liquid crystaldisplay device of the present invention;

FIGS. 12A and 12B each show a top view of a liquid crystal displaydevice of the present invention;

FIGS. 13A to 13D each show a manufacturing step of a liquid crystaldisplay device using a liquid crystal dropping method of the presentinvention;

FIGS. 14A and 14B each show a manufacturing step of a liquid crystaldisplay device using a liquid crystal dropping method of the presentinvention;

FIGS. 15A and 15B each show a manufacturing step of a liquid crystaldisplay device using a liquid crystal dropping method of the presentinvention;

FIGS. 16A and 16B each show a manufacturing step of a liquid crystaldisplay device using a liquid crystal dropping method of the presentinvention;

FIGS. 17A and 17B each show a manufacturing step of an EL display deviceof the present invention;

FIG. 18 shows a manufacturing step of an EL display device of thepresent invention;

FIG. 19 shows a manufacturing step of an EL display device of thepresent invention;

FIG. 20 shows a manufacturing step of an EL display device of thepresent invention;

FIGS. 21A to 21D each show an example of an electronic appliance towhich the present invention is applied;

FIGS. 22A to 22D each show an example of an electronic appliance towhich the present invention is applied;

FIGS. 23A to 23E each show a manufacturing step of a semiconductordevice of the present invention; and

FIGS. 24A and 24B each show a manufacturing step of a semiconductordevice of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In this embodiment mode, a structure of a semiconductor device of thepresent invention and a manufacturing method thereof are described withreference to FIG. 1 and FIGS. 3A to 3D.

First, a gate electrode 102 is formed over a substrate 101 (see FIG.3A). In this embodiment mode, a glass substrate is used as the substrate101, and the gate electrode 102 is formed over the glass substrate usinga tungsten film.

Although glass is used for the substrate 101 in this embodiment mode, asa substrate with low heat resistance, plastic, paper to which aplanarization treatment is performed on surfaces thereof, papers towhich a plastic treatment is performed on surfaces thereof, and the likecan be used, besides glass. Also, to form the gate electrode 102, asingle layer film such as a polycrystalline silicon (poly-Si) film, analuminum (Al) film, a titanium (Ti) film, a tantalum (Ta) film, or amolybdenum (Mo) film; or a stacked layer film that is a combinationthereof can be used besides the tungsten (W) film.

Subsequently, a gate insulating film 103 is formed over the gateelectrode 102 (see FIG. 3B). In this embodiment mode, a silicon oxidefilm containing nitrogen is used as the gate insulating film 103.

As the gate insulating film 103, a single layer film such as a siliconoxide film, a silicon nitride film, or an aluminum oxide film; or astacked layer film that is a combination thereof can be used besides thesilicon oxide film containing nitrogen.

Next, first source or drain electrodes 104 are formed over the gateinsulating film 103 (see FIG. 3C). In this embodiment mode, a titaniumfilm is used as the first source or drain electrodes 104. A titaniumfilm can reduce the difference in contact potential between the firstsource or drain electrodes 104 and an island-shaped semiconductor film105. Also, by forming the first source or drain electrodes 104 to bethin, coverage by the island-shaped semiconductor film 105 that issubsequently formed can be improved.

Note that the same material as that of the gate electrode 102 can beused for the first source or drain electrodes 104. That is, as the firstsource or drain electrodes 104, a single layer film such as a tungstenfilm, a polycrystalline silicon film, an aluminum film, a tantalum film,or a molybdenum film; or a stacked layer film that is a combinationthereof can be used besides the titanium film.

The island-shaped semiconductor film 105, for example a zinc oxide (ZnO)film is formed over the first source or drain electrodes 104 (see FIG.3D).

As the island-shaped semiconductor film 105, an inorganic semiconductorfilm such as a silicon (Si) film, a silicon germanium (SiGe) film, agallium arsenic (GaAs) film, a gallium nitride (GaN) film, an indiumoxide (InO_(x)) film, a tin oxide (SnO₂) film, an indium phosphorus(InP) film, an indium nitride (InN) film, a cadmium sulfide (CdS) film,or a cadmium telluride (CdTe) film; or an organic semiconductor filmsuch as a pentacene film, or an oligothiophene film can be used otherthen the zinc oxide film.

Next, second source or drain electrodes 106 are formed over theisland-shaped semiconductor film 105. Although a stacked layer film ofan aluminum film and a titanium film is formed as the second source ordrain electrodes 106 in this embodiment mode, a similar material tothose of the gate electrode 102 and the first source or drain electrodes104 may be used to form the second source or drain electrodes 106. Thatis, as the second source or drain electrodes 106, a single layer filmsuch as a titanium film, a tungsten film, a polycrystalline siliconfilm, an aluminum film, a tantalum film, or a molybdenum film can beused besides the stacked layer film of an aluminum film and a titaniumfilm; or a stacked layer film that is a combination thereof can be used,except for the combination of an aluminum film and a titanium film.

In the present invention, thicknesses of the first source or drainelectrodes 104 that are formed under the island-shaped semiconductorfilm 105 can be thinner than thicknesses of the second source or drainelectrodes 106 that are fainted over the island-shaped semiconductorfilm 105.

By making the thickness of the first source or drain electrodes 104thin, coverage of the island-shaped semiconductor film 105 can befavorable.

Further, by making the coatability of the island-shaped semiconductorfilm 105 favorable, TFT characteristics can be improved since a crack ina step portion and distortion due to stress concentration can besuppressed.

Also, the thickness of the first source or drain electrodes 104 can beformed to be a thickness that is half of that of the island-shapedsemiconductor film 105 to a thickness that is the same as that of theisland-shaped semiconductor film 105.

The second source or drain electrodes 106 are in contact with the firstsource or drain electrodes 104, and they each form a source electrode ora drain electrode. Also, regions in the island-shaped semiconductor filmeach serving as a source region or a drain region are sandwiched betweenthe first source or drain electrodes 104 and the second source or drainelectrodes 106.

In the formation of the second source or drain electrodes 106, the samemask as the mask used to etch the first source or drain electrodes 104can be used to etch the second source or drain electrodes 106.Consequently, the number of masks is not increased.

Also, by sandwiching the regions in the island-shaped semiconductor film105 each serving as a source region or drain region between the firstsource or drain electrodes 104 and the second source or drain electrodes106, contact resistance between the regions each serving as a sourceregion or drain region and wirings can be reduced and the thickness ofthe second source or drain electrodes 106 can be formed to be thick;consequently wiring resistance can be reduced as well.

In this manner, the source region and the drain region of thesemiconductor device of this embodiment mode each have a structure ofbeing sandwiched from above and below by the source or drain electrodes;therefore, contact resistance of the source or drain electrodes and thesemiconductor film can be reduced. Also, since there are two layers ofthe source or drain electrodes, wiring resistance can also be reduced.Further, since the first source or drain electrodes that are formedunder the semiconductor film can be formed to be thin, coverage of thesemiconductor film can be improved to improve TFT characteristics, andreliability of the TFT can also be improved.

Embodiment 1

In this embodiment, a semiconductor device of the present invention anda manufacturing method thereof are described with reference to FIG. 1,and FIGS. 23A to 24B.

First, a first conductive film is formed over the substrate 101, and thegate electrode 102 is formed using the first conductive film (see FIG.23A). In this embodiment, a glass substrate is used as the substrate101, and a tungsten film is formed over the glass substrate so as tohave a thickness of 100 nm to 200 nm, for example 150 nm, using asputtering apparatus. Then, a resist serving as an etching mask isformed over the tungsten film using a light exposure apparatus, aninkjet apparatus, or the like. Subsequently, the tungsten film is etchedusing a dry etching apparatus to form the gate electrode 102.

Although glass is used for the substrate 101 in this embodiment, as asubstrate with low heat resistance, plastic, paper, or the like can beused, besides glass. Also, as the first conductive film for forming thegate electrode 102, a single layer film of a polycrystalline silicon(poly-Si) film, an aluminum (Al) film, a titanium (Ti) film, a tantalum(Ta) film, a molybdenum (Mo) film, or the like; or a stacked layer filmthat is a combination thereof can be used besides the tungsten (W) film.

Subsequently, the gate insulating film 103 is formed over the gateelectrode 102 (see FIG. 23B). In this embodiment, as the gate insulatingfilm 103, a silicon oxide film containing nitrogen is formed to have athickness of 50 nm to 200 nm, for example 100 nm, using a CVD apparatusor the like.

As the gate insulating film 103, a single layer film of a silicon oxidefilm, a silicon nitride film, an aluminum oxide film, or the like; or astacked layer film that is a combination thereof can be used, besidesthe silicon oxide film containing nitrogen.

Next, a second conductive film 111 is formed over the gate insulatingfilm 103 (see FIG. 23C), and the first source or drain electrodes 104are formed using the second conductive film 111 (see FIG. 23D). In thisembodiment, as the second conductive film 111, a titanium film is formedto have a thickness of 50 nm to 100 nm, for example 50 nm, using asputtering apparatus. When a titanium film is used, the difference incontact potential between the second conductive film 111 and thesemiconductor film 112 can be reduced. Also, by making a thickness ofthe second conductive film 111 to be a thickness that is about half ofthat of the semiconductor film 112 to a thickness that is about the sameas that of the semiconductor film 112 (50 nm in this embodiment),coverage by the semiconductor film 112 that is subsequently formed canbe favorable. Then, a resist serving as an etching mask is formed usinga light exposure apparatus, an inkjet apparatus, or the like, and thenetching is performed using a dry etching apparatus to form the firstsource or drain electrodes 104.

Note that for the second conductive film 111, the same material as thatof the first conductive film can be used. That is, as the secondconductive film 111, a single layer film such as a tungsten film, apolycrystalline silicon film, an aluminum film, a tantalum film, or amolybdenum film; or a stacked layer film that is a combination thereofcan be used besides the titanium film.

As the semiconductor film 112, a zinc oxide (ZnO) film for example isformed over the first source or drain electrodes 104 so as to have athickness of 50 nm to 200 nm, for example 100 nm, by a sputtering method(see FIG. 23E). As described above, it is favorable that the thicknessof the semiconductor film 112 is a thickness that is about the same asthat of the first source or drain electrodes 104 to a thickness that isabout twice that of the first source or drain electrodes 104.Subsequently, a resist is formed using a light exposure apparatus, aninkjet apparatus, or the like, and the island-shaped semiconductor film105 is formed using a dry etching apparatus or by wet etching using ahydrofluoric acid aqueous solution or the like (see FIG. 24A).

As the semiconductor film 112, an inorganic semiconductor film such as asilicon (Si) film, a silicon germanium (SiGe) film, a gallium arsenic(GaAs) film, a gallium nitride (GaN) film, an indium oxide (InO_(x))film, a tin oxide (SnO₂) film, an indium phosphorus (InP) film, anindium nitride (InN) film, a cadmium sulfide (CdS) film, or a cadmiumtelluride (CdTe) film; or an organic semiconductor film such as apentacene film or an oligothiophene film can be used other than the zincoxide film. Note that, if necessary, the semiconductor film 112 may bedoped with phosphorus (P), arsenic (As) or the like which is an impurityimparting n-type conductivity. Also, the semiconductor film may be dopedwith boron (B) or the like which is an impurity imparting p-typeimpurity, if necessary.

Subsequently, a third conductive film 113 is formed over theisland-shaped semiconductor film 105 (see FIG. 24B). In this embodiment,a stacked layer film of an aluminum film and a titanium film is formedas the third conductive film 113. A thickness of the aluminum film is100 nm to 300 nm, for example, 150 nm, and a thickness of the titaniumfilm is 50 nm to 100 nm, for example 50 nm. Then, a resist serving as anetching mask is formed using a light exposure apparatus, an inkjetapparatus, or the like, and the second source or drain electrodes 106are formed using a dry etching apparatus (see FIG. 1).

Even if the thickness of the first source or drain electrodes 104 isthin, the thickness of the second source or drain electrodes 106 can beformed to be thick; therefore, contact resistance of the source or drainelectrodes as a whole can be reduced.

The second source or drain electrodes 106 are in contact with the firstsource or drain electrodes 104, and they each form a source electrode ordrain electrode. Also, regions in the island-shaped semiconductor film105 each serving as a source region or a drain region are sandwichedbetween the first source or drain electrodes 104 and the second sourceor drain electrodes 106.

Note that the third conductive film 113 can be formed using a similarmaterial to that of the first conductive film and the second conductivefilm 111. That is, as the third conductive film 113, a single layer filmsuch as a titanium film, a tungsten film, a polycrystalline siliconfilm, an aluminum film, a tantalum film, or a molybdenum film can beused besides the stacked layer film of an aluminum film and a titaniumfilm; or a stacked layer film that is a combination thereof can be used,except for the combination of an aluminum film and a titanium film.

In the formation of the second source or drain electrodes 106, the samemask as the mask used to etch the first source or drain electrodes 104can be used to etch the second source or drain electrodes 106.Consequently, the number of masks is not increased.

Also, by sandwiching the regions in the island-shaped semiconductor film105 each serving as a source region or drain region between the firstsource or drain electrodes 104 and the second source or drain electrodes106, contact resistance between the regions each serving as a sourceregion or drain region and wirings can be reduced and the thickness ofthe second source or drain electrodes 106 can be formed to be thick;consequently, wiring resistance can be reduced as well.

Embodiment 2

In this embodiment, a result of calculating a transistor characteristicof a TFT of the present invention is described with reference to FIG. 4.Note that a structure of the TFT calculated in this embodiment isequivalent to that shown in FIG. 1. Various conditions set in thisembodiment are listed below.

Length between the source electrode and the drain electrode (channellength) L: 3 μm

Width of each of the source electrode and the drain electrode (channelwidth) W: 1 μm

Semiconductor layer: silicon (Si) film

Thickness of the semiconductor layer: 100 nm

Impurity contained in the semiconductor layer: phosphorus (P)

Density of the impurity contained in the semiconductor layer: 1×10¹⁵cm⁻³

Gate insulating film: silicon oxide film

Thickness of the gate insulating film: 100 nm

For the calculation, a device simulator by Integrated SystemsEngineering (ISE) is used. For a physical model, an electrical fieldeffect is considered for mobility, and silicon which is thesemiconductor film is assumed to be ideal crystals without a defect.

In FIG. 4, a line interrupted by a single dot (hereinafter called“curved line 1”) indicates a calculation result of a Vg-Id curved linein a case where source or drain electrodes and the semiconductor layerare in contact only in an upper portion of the semiconductor layer, andform ohmic contact without contact resistance.

A dotted line (hereinafter called “curved line 2”) indicates acalculation result of the Vg-Id curved line in a case where the sourceor drain electrodes and the semiconductor layer are in contact only inthe upper portion of the semiconductor layer, and where 0.2 eV isapplied as a Schottky barrier.

A solid line (hereinafter called “curved line 3”) indicates acalculation result of the Vg-Id curved line of a structure of a TFT ofthe present invention. In other words, curved line 3 is a calculationresult of the Vg-Id curved line in a case where the source or drainelectrodes and the semiconductor layer are in contact in the upperportion and a lower portion of the semiconductor layer, and where 0.2 eVis applied as Schottky barriers thereof.

The case of curved line 1 in which there is ohmic contact withoutcontact resistance is equivalent to a case where the semiconductor layerand the electrodes are heated to eliminate contact resistance when a TFTis actually manufactured. On the other hand, the case of curved line 2and curved line 3 where there is a Schottky barrier is equivalent to acase where heating of the semiconductor layer and the electrodes is notperformed and resistance exists.

Curved line 1 is an ideal case where there is not contact resistancebetween the semiconductor layer and the electrodes. However, normally,contact resistance occurs between the semiconductor layer and theelectrodes and this decreases on current and mobility. Curved line 2shows a case where there is contact resistance equivalent to a Schottkybarrier of 0.2 eV between the semiconductor layer and the electrodes.When curved line 2 is examined, it is apparent that on current isdecreased due to contact resistance. To reduce contact resistance, aheat treatment at high temperature may be performed; however, when aplastic film or paper is used as a substrate, it is difficult to reducecontact resistance by a heat treatment.

As it is clear from comparing curved line 2 and curved line 3, in thecase where the semiconductor layer and the electrodes are in contact inboth the upper portion and the lower portion of the semiconductor layer(curved line 3), contact resistance is reduced and on current is abouttwice as large as that of the case where they are in contact only in theupper portion of the semiconductor layer (curved line 2). Therefore, byusing the present invention, on current of a TFT can be doubled withouta heat treatment at high temperature.

Also, although on current can be doubled if channel width is doubled, asize of a device increases by that much, and integration density of thedevice decreases. However, if a TFT in which a source region and a drainregion are sandwiched from above and below by wirings is used, oncurrent can be doubled without changing the size of an element.

Embodiment 3

This embodiment describes an example of manufacturing a semiconductordevice of the present invention by an inkjet method, with reference toFIGS. 5A to 5E.

A gate electrode 202 is formed over a substrate 201 using a conductivepaste, by an inkjet method (see FIG. 5A).

As the conductive paste, a conductive paste containing a metal materialsuch as silver (Ag), gold (Au), copper (Cu) or nickel (Ni); or aconductive carbon paste can be used.

A gate insulating film 203 is formed over the substrate 201 and the gateelectrode 202 by an inkjet method (see FIG. 5B).

As a material for the gate insulating film 203, a composition containinga photosensitizer may be used. For example, a positive type resist madeof a novolac resin and a naphthoquinone diazide compound, or a negativetype resist made by dissolving or dispersing a base resin,diphenylsilanediol, an acid generator, and the like in a solvent isused. As the solvent, esters such as butyl acetate or ethyl acetate;alcohols such as isopropyl alcohol or ethyl alcohol; or organic solventssuch as methyl ethyl ketone or acetone is used. Concentration of thesolvent may be appropriately set depending on the type of resist.

First source or drain electrodes 204 are formed over the gate insulatingfilm 203. The first source or drain electrodes 204 may be formed using asimilar material and process to those of the gate electrode 202.

Subsequently, an island-shaped semiconductor film 205 is formed. In thisembodiment, the island-shaped semiconductor film 205 is formed using anorganic semiconductor material by an inkjet method. For the organicsemiconductor material, either a low molecular compound or a highmolecular compound can be used as long as it is an organic material thathas a carrier transporting property, and by which modulation in carrierdensity can occur by an electrical field effect.

The type of the organic semiconductor material is not particularlylimited; however, a polycyclic aromatic compound, a conjugated doublebond compound, a metallophthalocyanine complex, a charge transfercomplex, condensed ring tetracarboxylic diimides, oligothiophenes,fullerenes, a carbon nanotube, and the like are given. For example,polypyrrole, polythiophene, poly(3alkylthiophene), polyisothianaphthene,polythienylenevinylene, poly(p-phenylenevinylene), polyaniline,polydiacetylene, polyazulene, polypyrene, polycarbazole,polyselenophene, polyfuran, poly(p-phenylene), polyindole,polypyridazine, naphthacene, hexacene, heptacene, pyrene, chrysene,perylene, coronene, terrylene, ovalene, quaterrylene, circumanthracene,triphenodioxazine, triphenodiriazine, hexacene-6,15-quinone,polyvinylcarbazole, polyphenylenesulfide, polyvinylenesulfide,polyvinylpyridine, naphthalenetetracarboxylic diimide,anthracenetetracarboxylic diimide, C₆₀, C₇₀, C₇₆, C₇₈, C₈₄, and aderivative thereof can be used. As a specific example of thesematerials, there are tetracene, pentacene, sexithiophene (6T),α,ω-dihexyl-sexithiophene, copper phthalocyanine,5-benzylidene-2,4-dioxotetrahydro-1,3-thiazole,2,2′-bi(dithieno[3,2-b:2′,3′-d]thiophene),bis-(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiol),di(4-biphenyl)-α-thiophene, 2,5-di(4-biphenylyl)thiophene,di(4-biphenyl)-α-bithiophene, 5,5′-di(4-biphenylyl)-2,2-bithiophene,di(4-biphenyl)-α-terthiophene,5,5″-di(4-biphenylyl)-2,2′:5′,2″-terthiophene,di(4-biphenyl)-α-quaterthiophene,5,5′″-di(4-biphenylyl)-α-quaterthiophene, dihexylanthradithiophene,2,8-dihexylanthra[2,3-b:6,7-b′]dithiophene, rubrene,dihexyl-α-tetrathiophene, 5,5′″-di(4-biphenylyl)-α-quaterthiophene,dihexyl-α-pentathiophene, 5,5′″-dihexyl-α-quinquethiophene,poly(2,5-thienylenevinylene) (abbreviation: PTV),poly(3-hexylthiophene-2,5-diyl) (abbreviation: P3HT), orpoly(9,9′-dioctyl-fluorene-co-bithiophene) (abbreviation: F8T2) which isgenerally considered as a p-type semiconductor;7,7,8,8,-tetracyanoquinodimethane (abbreviation: TCNQ),3,4,9,10-perylenetetracarboxylicdianhydride (abbreviation: PTCDA),1,4,5,8-naphthalenetetracarboxylicdianhydride (abbreviation: NTCDA),9,9,10,10,-tetracyano-2,6-naphthaquinodimethane (abbreviation: TCNNQ),N,N′-dioctyl-3,4,9,10-perylenetetracarboxylicdiimide (abbreviation:PTCDI-C8H), copper16phthalocyaninefluoride (abbreviation: F₁₆CuPc),N,N′-2,2,3,3,4,4,5,5,6,6,7,7,7-di15hexylfluoride-1,4,5,8-naphthalenetetracarboxylicdiimide (abbreviation: NTCDI-C8F), α,ω-bis(perfluorohexyl)sexithiophene(abbreviation: DFH-6T),3′,4′-dibutyl-5,5″-bis(dicyanomethylene)-5,5″-dihydro-2,2′:5′,2″-terthiophene)(abbreviation: DCMT), or methanofullerene[6,6]-phenyl C₆₁ butyric acidmethyl ester (abbreviation: PCBM) which is generally considered as ann-type semiconductor; and the like.

Note that p-type or n-type characteristic of the organic semiconductoris not inherent to the substance, but depends on a relationship with anelectrode to which carriers are injected and an intensity of an electricfield during the injection. Although a substance has its own tendency ofbecoming a p-type or an n-type, the organic semiconductor can be used asa p-type semiconductor or an n-type semiconductor.

Subsequently, second source or drain electrodes 206 are formed to be incontact with the island-shaped semiconductor film 205 and the firstsource or drain electrodes 204. The second source or drain electrodes206 may be formed using a similar material and process to those of thefirst source or drain electrodes 204.

In the above manner, the semiconductor device of this embodiment ismanufactured. Since the semiconductor device of this embodiment isformed by an inkjet method, a manufacturing process can be shortened,and manufacturing cost can be kept low.

Note that although the semiconductor device of this embodiment is formedby an inkjet method, manufacturing methods described in the embodimentmode and other embodiments may be combined if necessary, and it may bethat an inkjet method is used just in a necessary step.

In this manner, a source region and a drain region of the semiconductordevice of this embodiment each have a structure of being sandwiched fromabove and below by source or drain electrodes; therefore, contactresistance of the source or drain electrodes and the semiconductor filmcan be reduced. Also, since there are two layers of the source or drainelectrodes, wiring resistance can also be reduced. Further, since thefirst source or drain electrodes that are formed under the semiconductorfilm can be formed to be thin, even in a case of using an organicsemiconductor film, coverage thereof can be improved to improve TFTcharacteristics, and reliability of the TFT can also be improved.

Embodiment 4

In this embodiment, an example of forming a semiconductor film addedwith an impurity imparting one conductivity type between anisland-shaped semiconductor film and second source or drain electrodeswhich are formed over the island-shaped semiconductor film, is describedwith reference to FIGS. 6A to 6D.

First, formation of up to the island-shaped semiconductor film 105 shownin FIG. 6A is performed according to the embodiment mode andEmbodiment 1. Subsequently, a semiconductor film 121 added with animpurity imparting one conductivity type is formed by a CVD method orthe like (see FIG. 6A).

For the impurity imparting one conductivity type, phosphorus (P) orarsenic (As) which is an impurity imparting n-type conductivity may beused when forming an n-channel TFT and boron (B) which is an impurityimparting p-type impurity may be used when forming a p-channel

Subsequently, etching is performed on the semiconductor film 121 addedwith the impurity imparting one conductivity type using a resist that isan etching mask, to form island-shaped impurity semiconductor films 122each serving as a source region or drain region (see FIG. 6B). Also, achannel forming region is formed in a region of the island-shapedsemiconductor film 105 that is between a source region and a drainregion.

Next, a conductive film 123 is formed over the first source or drainelectrodes 104 and the island-shaped impurity semiconductor films 122(see FIG. 6C). As the conductive film 123, a single layer film of atitanium (Ti) film, a tungsten (W) film, a polycrystalline silicon(poly-Si) film, an aluminum (Al) film, a tantalum (Ta) film, amolybdenum (Mo) film, or the like; or a stacked layer film that is acombination thereof can be used.

Subsequently, the conductive film 123 is etched to form second source ordrain electrodes 124 (see FIG. 6D). In the above manner, thesemiconductor device of this embodiment is formed.

In this manner, the source region and the drain region of thesemiconductor device of this embodiment each have a structure of beingsandwiched from above and below by the source or drain electrodes;therefore, contact resistance of the source or drain electrodes and thesemiconductor film can be reduced. Also, since there are two layers ofthe source or drain electrodes, wiring resistance can also be reduced.Further, since the first source or drain electrodes that are formedunder the semiconductor film can be formed to be thin, coverage of thesemiconductor film can be improved to improve TFT characteristics, andreliability of the TFT can also be improved.

Embodiment 5

In this embodiment, an example of manufacturing a liquid crystal displaydevice using the present invention is described with reference to FIGS.7A to 12B.

First, based on the description in the embodiment mode and Embodiments 1and 4, n-channel TFTs 355 and 357, and a p-channel TFT 356 are formedover a substrate 301 (see FIG. 7A). The n-channel TFT 355 and thep-channel TFT 356 form a CMOS circuit 358.

The n-channel TFT 355 includes a gate electrode 302, a gate insulatingfilm 305, first source or drain electrodes 311 and 312, an island-shapedsemiconductor film 317 serving as a channel forming region,island-shaped impurity semiconductor films 321 and 322 each serving as asource region or drain region, and second source or drain electrodes 331and 332.

The p-channel TFT 356 includes a gate electrode 303, the gate insulatingfilm 305, the first source or drain electrode 312 and a first source ordrain electrode 313, an island-shaped semiconductor film 318 serving asa channel forming region, island-shaped impurity semiconductor films 323and 324 each serving as a source region or drain region, and the secondsource or drain electrode 332, and a second source or drain electrode333.

The n-channel TFT 355 and the p-channel TFT 356 are electricallyconnected by the first source or drain electrode 312 and the secondsource or drain electrode 332, to form the CMOS circuit 358.

The n-channel TFT 357 includes a gate electrode 304, the gate insulatingfilm 305, first source or drain electrodes 314 and 315, an island-shapedsemiconductor film 319 serving as a channel forming region,island-shaped impurity semiconductor films 325 and 326 each serving as asource region or drain region, and second source or drain electrodes 334and 335.

As the substrate 301, a similar substrate to the substrate 101 of theembodiment mode and Embodiment 1 may be used. Also, the gate electrodes302 to 304, the gate insulating film 305, the first source or drainelectrodes 311 to 315, the island-shaped semiconductor films 317 to 319,and the second source or drain electrodes 331 to 335 may be formed withreference to the gate electrode 102, the gate insulating film 103, thefirst source or drain electrodes 104, the island-shaped semiconductorfilm 105, and the second source or drain electrodes 106 of theembodiment mode and Embodiment 1, respectively.

Further, each of the island-shaped impurity semiconductor films 321 and322 of the n-channel 355 as well as the island-shaped impuritysemiconductor films 325 and 326 of the n-channel TFT 357 may be formedby forming a semiconductor film added with phosphorus (P) or arsenic(As) which is an impurity imparting n-type conductivity. On the otherhand, each of the island-shaped impurity semiconductor films 323 and 324of the p-channel TFT 356 may be formed by forming a semiconductor filmadded with boron (B) which is an impurity imparting p-type conductivity.

Also, although the TFT 357 functions as a pixel TFT in the liquidcrystal display device of this embodiment, a p-channel TFT instead of ann-channel TFT may be used as the pixel TFT if necessary. In that case,each of the island-shaped impurity semiconductor films 325 and 326 ofthe TFT 357 may be formed using a semiconductor film added with a p-typeimpurity instead of a semiconductor film added with an n-type impurity.

Subsequently, a first interlayer insulating film 341 is formed over then-channel TFT 355, the p-channel TFT 356, and the n-channel TFT 357.

As the first interlayer insulating film 341, an insulating filmcontaining silicon, for example a silicon oxide film, a silicon nitridefilm, or a silicon oxide film containing nitrogen; or a stacked layerfilm thereof is formed by a sputtering method. Of course, the firstinterlayer insulating film 341 is not limited to the silicon oxide filmcontaining nitrogen, the silicon nitride film, or the stacked layer filmthereof, and a single layer or a stacked layer of another insulatingfilm containing silicon may be used. Further, if possible, the firstinterlayer insulating film 341 may be formed using the foregoingmaterials by a plasma CVD method.

In this embodiment, after introducing the impurity, a silicon oxide filmcontaining nitrogen is formed to have a thickness of 50 nm by asputtering method. At that time, laser irradiation may be performed overthe silicon oxide film containing nitrogen to activate the impurity.

For laser crystallization, a continuous wave laser or a pulsed laserwith a repetition rate of 10 MHz or more, preferably 80 MHz or more as apseudo CW laser can be used.

Specifically, the following and the like can be given as the continuouswave laser: an Ar laser, a Kr laser, a CO₂ laser, a YAG laser, a YVO₄laser, a forsterite (Mg₂SiO₄) laser, a YLF laser, a YAlO₃ laser, a GdVO₄laser, a Y₂O₃ laser, an alexandrite laser, a Ti:sapphire laser, a heliumcadmium laser, and a laser of which a medium is a polycrystalline(ceramic) YAG, Y₂O₃, YVO₄, YAlO₃, or GdVO₄, added with one or more ofNd, Yb, Cr, Ti, Ho, Er, Tm, and Ta as a dopant.

Also, as the pseudo CW laser, a pulsed laser such as the following canbe used if pulse oscillation has a repetition rate of 10 MHz or more,preferably 80 MHz or more: an Ar laser, a Kr laser, an excimer laser, aCO₂ laser, a YAG laser, a Y₂O₃ laser, a YVO₄ laser, a forsterite(Mg₂SiO₄) laser, a YLF laser, YAlO₃ laser, a GdVO₄ laser, an alexandritelaser, a Ti:sapphire laser, a copper vapor laser, a gold vapor laser, ora laser of which a medium is a polycrystalline (ceramic) YAG, Y₂O₃,YVO₄, YAlO₃, or GdVO₄, added with one or more of Nd, Yb, Cr, Ti, Ho, Er,Tm, and Ta as a dopant.

Such a pulsed laser eventually exhibits an effect equivalent to that ofa continuous wave laser when repetition rate is increased.

Subsequently, a silicon nitride film is formed to have a thickness of 50nm by a sputtering method, and further, a silicon oxide film containingnitrogen is formed to have a thickness of 60 nm. A stacked layer film ofthese silicon oxide film containing nitrogen, silicon nitride film, andsilicon oxide film containing nitrogen is the first interlayerinsulating film 341.

Next, the substrate and everything formed thereover are heated at 410°C. for 1 hour, and hydrogenation is performed by releasing hydrogen fromthe silicon nitride film.

Subsequently, a second interlayer insulating film 342 functioning as aplanarization film is formed covering the first interlayer insulatingfilm 341 (see FIG. 7B).

As the second interlayer insulating film 342, a photosensitive ornon-photosensitive organic material (polyimide, acrylic, polyamide,polyimide amide, a resist, or benzocyclobutene), siloxane, or a stackedlayer structure thereof can be used. As the organic material, apositive-type photosensitive organic resin or a negative-typephotosensitive organic resin can be used.

Note that siloxane has a skeleton that is structured by a bond betweensilicon (Si) and oxygen (O), and uses an organic group containing atleast hydrogen for a substituent. Also, for the substituent, a fluorogroup may be used. Further, an organic group containing at leasthydrogen, and a fluoro group may be used for the substituent.

In this embodiment, siloxane is formed-as the second interlayerinsulating film 342 by a spin coating method.

Etching is performed on the first interlayer insulating film 341 and thesecond interlayer insulating film 342 to form contact holes in the firstinterlayer insulating layer 341 and the second interlayer insulatingfilm 342 that reach the second source or drain electrodes 331, 333, 334and 335.

Note that a third interlayer insulating film may be formed over thesecond interlayer insulating film 342, and the contact holes may beformed in the first interlayer insulating film to the third interlayerinsulating film. As the third interlayer insulating film, a film thatdoes not easily allow permeation of moisture, oxygen, or the likecompared to other insulating films is used. Typically, a silicon nitridefilm, a silicon oxide film, a silicon nitride film containing oxygen, asilicon oxide film containing nitrogen, a thin film mainly containingcarbon (for example, a DLC film, or a CN film), or the like obtained bya sputtering method or a CVD method can be used.

A third conductive film is formed over the second interlayer insulatingfilm 342 via a contact hole, and an electrode or wiring 345, anelectrode or wiring 346, and electrode or wiring 347, and an electrodeor wiring 348 (also collectively referred to as electrodes or wirings345 to 348) are formed using the third conductive film.

In this embodiment, a metal film is used for the third conductive film.As the metal film, a film made of an element of aluminum (Al), titanium(Ti), molybdenum (Mo), tungsten (W), or silicon (Si), or an alloy filmusing such an element may be used. In this embodiment, a titanium (Ti)film, a titanium nitride (TiN) film, a silicon-aluminum alloy (Al—Si)film, and a titanium (Ti) film with thicknesses of 60 nm, 40 nm, 300 nm,and 100 nm, respectively, are stacked, and then subsequently etched intoa desired shape to form the electrodes or wirings 345 to 348.

Also, these electrodes or wirings 345 to 348 may be formed of at leastone element selected from nickel, cobalt, and iron, or an aluminum alloyfilm containing carbon. With such an aluminum alloy film, mutualdiffusion of silicon and aluminum can be prevented even if it comes intocontact with silicon. Also, with such an aluminum alloy film, anoxidation-reduction reaction does not occur even if it comes intocontact with a transparent conductive film, for example an indium tinoxide (ITO) film; therefore, they can come into direct contact with eachother. Also, since such an aluminum alloy film has low specificresistance as well as excellent heat resistance, it is useful as awiring material.

Also, in forming the electrodes or wirings 345 to 348, an electrode anda wiring may be formed in the same step with the same material, or theelectrode and the wiring may be formed separately and then connected.

Subsequently, a third interlayer insulating film 351 is formed over thesecond interlayer insulating film 342 and the electrodes or wirings 345to 348. Note that the third interlayer insulating film 351 can be formedusing a similar material to that of the second interlayer insulatingfilm 342.

Note that when a film that does not easily allow permeation of moisture,oxygen, or the like compared to other insulating films is formed as thethird interlayer insulating film over the second interlayer insulatingfilm 342, the insulating film 351 serves as a fourth interlayerinsulating film.

Subsequently, a resist mask is formed using a photomask, and a portionof the third interlayer insulating film 351 is removed by dry etching toform a contact hole. In forming this contact hole, carbon tetrafluoride(CFA oxygen (O₂), and helium (He) are used as an etching gas with a flowamount of 50 sccm, 50 sccm, and 30 sccm, respectively. Note that thebase of the contact hole reaches the conductive film 348 functioning asan electrode or wiring.

Subsequently, after removing the resist mask, a conductive film isformed over the entire surface. Then, a pixel electrode 352 that iselectrically connected to the conductive film 348 functioning as anelectrode or wiring is formed using a second conductive film (see FIG.8B).

When manufacturing a transmissive type liquid crystal display device,the pixel electrode 352 is formed using a transparent conductive filmsuch as indium tin oxide (ITO), indium tin oxide containing siliconoxide, zinc oxide (ZnO), or tin oxide (SnO₂).

On the other hand, when manufacturing a reflective type liquid crystaldisplay device, the pixel electrode 352 may be formed using a metalmaterial having a light reflective property such as Ag (silver), Au(gold), Cu (copper), W (tungsten), or Al (aluminum), by a sputteringmethod.

FIG. 10 shows an enlarged top view of a portion of a pixel portion 371including an n-channel TFT 357 serving as a pixel TFT. Further, FIG. 10shows a pixel electrode formation in progress, and shows a state inwhich a pixel electrode is formed in a pixel on the left side, but notin a pixel on the right side. In FIG. 10, a cross-sectional view along asolid line A-A′ corresponds to the cross-section of a pixel portion inFIG. 9, and the same reference numerals are used for portionscorresponding to FIGS. 7A to 8B.

A capacitor wiring 369 is provided for each pixel, and a storagecapacitor is formed including the pixel electrode 352 and the capacitorwiring 369 that overlaps with the pixel electrode 352, with the firstinterlayer insulating film 341 as a derivative.

Note that in this embodiment, in a region in which the pixel electrode352 and the capacitor wiring 369 overlap each other, the secondinterlayer insulating film 342 and the third interlayer insulating film351 are etched, and the storage capacitor is formed with the pixelelectrode 352, the first interlayer insulating film 341 and thecapacitor wiring 369. However, if the second interlayer insulating film342 and the third interlayer insulating film 351 can be used asderivatives, then etching is not necessary to be performed on the secondinterlayer insulating film 342 and the third interlayer insulating film351. In that case, the first interlayer insulating film 341, the secondinterlayer insulating film 342, and the third interlayer insulating film351 function as derivatives. Alternatively, it may be that only thethird interlayer insulating film 351 is etched, and the first interlayerinsulating film 341 and the second interlayer insulating film 342 may beused as derivatives.

By the foregoing step, a TFT substrate of a liquid crystal displaydevice in which the pixel TFT 357, the n-channel TFT 355 and thep-channel TFT 356 that structure the CMOS circuit 358, and the pixelelectrode 352 are formed over the substrate 301, is completed.

Next, an orientation film 359 is formed so as to cover the pixelelectrode 352. Note that a droplet discharging method, a screen printingmethod, or an offset printing method may be used to form the orientationfilm 359. Subsequently, a rubbing treatment is performed on a surface ofthe orientation film 359.

Then, over an opposing substrate 361, a color filter including acoloring layer 363, a light shielding layer (a black matrix) 362, and anovercoat layer 364 is provided. Further, an opposing electrode 365 madeof a transparent electrode or a reflective electrode is formed, as wellas an orientation film 366 thereover (see FIG. 9).

Subsequently, a sealant 381 of a closed pattern is formed by a dropletdischarging method so as to surround a region which overlaps with thepixel portion 371 including a pixel TFT (see FIG. 11A). Here, an exampleof drawing the sealant 381 of a closed pattern is shown because liquidcrystal is dropped; however, a dipping method (a pumping method) may beused in which a seal pattern having an opening portion is provided andthen liquid crystal is injected by a capillary phenomenon afterattaching the substrate 301.

Next, a liquid crystal 367 is dropped under reduced pressure so that airbubbles do not enter (see FIG. 11B), and the TFT substrate 301 and theopposing substrate 361 are attached together (see FIG. 11C). The liquidcrystal 367 is dropped one time or a plurality of times within a closedloop seal pattern. As an orientation mode of the liquid crystal 367, aTN mode is used in many cases, in which an alignment of liquid crystalmolecules twists by 90° from where light enters to where it exists. Whena liquid crystal display device of the TN mode is manufactured,substrates are attached together so that rubbing directions of thesubstrates are perpendicular to each other.

Note that space between a pair of substrates may be maintained bydispersing a spherical spacer, forming a columnar spacer made of aresin, or including a filler in the sealant 381. The columnar spacer isan organic resin material mainly containing at least one of acrylic,polyimide, polyimide amide, and epoxy; or an inorganic material of asingle layer film of one material selected from silicon oxide, siliconnitride, or silicon oxide containing nitrogen, or a stacked layer filmthereof.

Subsequently, the substrates are divided. In a case of obtaining aplurality of panels from the substrates, each panel is separated off. Ina case of obtaining one panel from the substrates, a division step canbe skipped by attaching an opposing substrate that is cut in advance(see FIG. 11D).

Then, an FPC (flexible printed circuit) is attached via an anisotropicconductive layer by a known technique. By the foregoing step, the liquidcrystal display device of this embodiment is completed. Further, ifnecessary, an optical film is attached. In a case of a transmissive typeliquid crystal display device, a polarizing plate is attached to each ofthe TFT substrate 301 and the opposing substrate 361.

A top view of the liquid crystal display device obtained by theforegoing step is shown in FIG. 12A, and an example of a top view ofanother liquid crystal display device is shown in FIG. 12B.

In FIG. 12A, reference numerals 301, 361, 371, 372, 373, 381, and 382denote the TFT substrate, the opposing substrate, the pixel portion, asource signal line driver circuit, a gate signal line driver circuit,the sealant, and an FPC, respectively. Note that liquid crystal isdischarged by a droplet discharging method, and the substrate 301 andthe opposing substrate 361 which are a pair are attached together by thesealant 381 under reduced pressure.

In FIG. 12B, reference numerals 301, 361, 372, 373, 371, 381 a, and 382denote the TFT substrate, the opposing substrate, the source signal linedriver circuit, the gate signal line driver circuit, the pixel portion,a first sealant, and the FPC, respectively. Note that liquid crystal isdischarged by a droplet discharging method, and the substrate 301 andthe opposing substrate 361 which are a pair are attached together by thefirst sealant 381 a and a second sealant 381 b. Since liquid crystal isnot required for the source signal line driver circuit 372 and the gatesignal line driver circuit 373, liquid crystal is held just in the pixelportion 371, and the second sealant 381 b is provided for reinforcementof the entire panel.

In this manner, the source region and the drain region of asemiconductor device of this embodiment have a structure of beingsandwiched from above and below by source or drain electrodes;therefore, contact resistance of the source or drain electrodes and thesemiconductor film can be reduced. Also, since there are two layers ofthe source or drain electrodes, wiring resistance can also be reduced.Further, since the first source or drain electrodes that are formedunder the semiconductor film can be formed to be thin, coverage of thesemiconductor film can be improved to improve TFT characteristics, andreliability of the liquid crystal display device as a whole can also beimproved.

Also, this embodiment can be freely combined with the embodiment modeand Embodiments 1 to 4, if necessary.

Embodiment 6

In this embodiment, an example of using a droplet discharging method fordropping liquid crystal is described. In this embodiment, amanufacturing example of obtaining four panels using a large areasubstrate 400 is shown with reference to FIGS. 13A to 16B.

FIG. 13A shows a cross-sectional view of a liquid crystal layerformation by a dispenser (or an inkjet) in progress, and a liquidcrystal material 404 is discharged, sprayed, or dropped from a nozzle408 of a droplet discharging apparatus 406 so as to cover a pixelportion 401 that is surrounded by a sealant 402. The droplet dischargingapparatus 406 is moved in the direction of an arrow in FIG. 13A. Notealthough that an example of moving the nozzle 408 is shown here, theliquid crystal layer may be formed by fixing the nozzle and moving thesubstrate.

Also, a perspective view is shown in FIG. 13B, which shows a scene wherethe liquid crystal material 404 is selectively discharged, sprayed, ordropped just in regions surrounded by the sealant 402, and a droppingsurface 405 is moved in a nozzle scanning direction 403.

Also, FIGS. 13C and 13D each show an enlarged cross-sectional view of aportion 409 surrounded by a dotted line in FIG. 13A. When viscosity ofthe liquid crystal material is high, the liquid crystal material isdischarged continuously, and sticks without a break as in FIG. 13C. Onthe other hand, when viscosity of the liquid crystal material is low,the liquid crystal material is discharged intermittently, and dropletsare dropped in a dot form as in FIG. 13D.

Note that in FIG. 13C, reference numerals 410 and 411 denote a pixel TFTformed according to the present invention and a pixel electrode,respectively. The pixel portion 401 includes pixel electrodes arrangedin a matrix farm and a switching element connected to the pixelelectrodes, and here, a bottom gate TFT and a storage capacitor are alsoincluded.

A panel manufacturing flow is described below with reference to FIGS.14A to 15B.

First, the first substrate 400 with an insulating surface over which thepixel portion 401 is formed is prepared′. For the first substrate 400,formation of an orientation film, rubbing treatment, dispersion of aspherical spacer or formation of a columnar spacer, formation of a colorfilter, or the like is performed in advance. For a detailedmanufacturing method thereof, refer to Embodiment 5.

Subsequently, as shown in FIG. 14A, the sealant 402 is formed in apredetermined position (a pattern surrounding the pixel portion 401)over the first substrate 400 by a dispenser apparatus or an inkjetapparatus under an inert gas atmosphere or under reduced pressure. Forthe sealant 402 which is semi-transparent, a material including a filler(6 μm to 24 μm in diameter) and that which has a viscosity of 40 Pa·s to400 Pa·s is used. Note that it is preferable that a material that doesnot dissolve in liquid crystal, which it comes into contact with later,is selected. As the sealant 402, an acrylic based light curable resin oran acrylic based heat curable resin may be used. Further, since it is asimple seal pattern, the sealant 402 can also be formed by a printingmethod.

Next, the liquid crystal material 404 is dropped in a region surroundedby the sealant 402 by an inkjet method (see FIG. 14B). As the liquidcrystal material 404, a known liquid crystal material having a viscosityby which discharging by an inkjet method is possible may be used. Also,since viscosity of the liquid crystal material 404 can be set byadjusting a temperature, it is suited for the inkjet method. By theinkjet method, a necessary amount of the liquid crystal material 404 canbe stored in the region surrounded by the sealant 402 without waste.

Subsequently, the first substrate 400 over which the pixel portion 401is provided and the second substrate 421 over which the opposingelectrode and the orientation film are provided are attached togetherunder reduced pressure so that air bubbles do not enter (see FIG. 15A).Here, ultraviolet irradiation or a heat treatment is performed at thesame time as attaching the first substrate 400 and the second substrate421 together, and the sealant 402 is cured. Note that the heat treatmentmay be performed in addition to ultraviolet irradiation.

Also, FIGS. 16A and 16B each show an example of an attaching apparatuscapable of ultraviolet irradiation or heat treatment during or afterattaching of substrates.

In FIGS. 16A and 16B, reference numerals 422, 423, 424, 428, and 429denote a first substrate supporting base, a second substrate supportingbase, a light transmitting window, a downside measuring plate, and anultraviolet light source, respectively. Note that in FIGS. 16A and 16B,the same reference numerals are used for portions corresponding to FIGS.13A to 15B

The downside measuring plate 428 includes a heater, and cures thesealant 402. Also, the second substrate supporting base 423 is providedwith the light transmitting window 424 and allows ultraviolet light andthe like from the light source 429 to pass through. Although not shownhere, a position aligning of a substrate is performed through the window424. Further, the second substrate 421 serving as an opposing substrateis cut into a desired size in advance, and fixed to the second substratesupporting base 423 by a vacuum chuck or the like. A state beforeattaching together the first substrate 400 and the second substrate 421is shown in FIG. 16A.

At the time of attachment, after moving down the first substratesupporting base 422 and the second substrate supporting base 423, thefirst substrate 400 and the second substrate 421 are attached togetherby pressure, and then cured by performing ultraviolet irradiation as itis. A state after attaching together the first substrate 400 and thesecond substrate 421 is shown in FIG. 16B.

Subsequently, the first substrate 400 is cut using a cutting apparatussuch as a scriber apparatus, a breaker apparatus, or a roll cutter (seeFIG. 15B). In this manner, four panels can be manufactured from onesubstrate. Then, an FPC is attached using a known technique.

Note that as each of the first substrate 400 and the second substrate421, a glass substrate or a plastic substrate can be used.

In this manner, in this embodiment, a semiconductor device of thepresent invention is referred to when manufacturing a liquid crystaldisplay device by a droplet discharging method. A source region and adrain region of the semiconductor device of the present invention eachhave a structure of being sandwiched from above and below by source ordrain electrodes; therefore, contact resistance of the source or drainelectrodes and a semiconductor film can be reduced. Also, since thereare two layers of the source or drain electrodes, wiring resistance canalso be reduced. Further, since first source or drain electrodes thatare formed under the semiconductor film can be formed to be thin,coverage of the semiconductor film can be improved to improve TFTcharacteristics, and reliability of the liquid crystal display device asa whole can also be improved.

Further, this embodiment can be freely combined with the embodiment modeand Embodiments 1 to 5, if necessary.

Embodiment 7

In this embodiment, an example of manufacturing a dual emission displaydevice using the present invention is described with reference to FIGS.17A to 20.

First, based on the embodiment mode and Embodiments 1 to 4, n-channelTFTs 561 and 562, and a p-channel TFT 563 are formed over a substrate501.

The n-channel TFT 561 includes a gate electrode 502, a gate insulatingfilm 505, first source or drain electrodes 511 and 512, an island-shapedsemiconductor film 517 serving as a channel forming region,island-shaped impurity semiconductor films 521 and 522 each serving as asource region or drain region, and second source or drain electrodes 531and 532.

The n-channel TFT 562 includes a gate electrode 503, the gate insulatingfilm 505, first source or drain electrodes 513 and 514, an island-shapedsemiconductor film 518 serving as a channel forming region,island-shaped impurity semiconductor film 523 and 524 each serving as asource region or drain region, and second source or drain electrodes 533and 534.

The p-channel 563 includes a gate electrode 504, the gate insulatingfilm 505, first source or drain electrodes 515 and 516, an island-shapedsemiconductor film 519 serving as a channel forming region,island-shaped impurity semiconductor films 525 and 526 each serving as asource region or drain region, and second electrode 535 and 536.

As the substrate 501, a similar substrate to the substrate 101 of theembodiment mode and Embodiment 1 may be used. Also, the gate electrodes502 to 504, the gate insulating film 505, the first source or drainelectrodes 511 to 516, the island-shaped semiconductor films 517 to 519,and the second source or drain electrodes 531 to 536 may be formed withreference to the gate electrode 102, the gate insulating film 103, thefirst source or drain electrodes 104, the island-shaped semiconductorfilm 105, and the second source or drain electrodes 106 of theembodiment mode and Embodiment 1, respectively.

Also, each of the island-shaped impurity semiconductor films 521 and 522of the n-channel TFT 561 as well as the island-shaped impuritysemiconductor films 523 and 524 of the n-channel TFT 562 may be formedby forming a semiconductor film to which phosphorus (P) or arsenic (As),which is an impurity imparting n-type, is added. On the other hand, eachof the island-shaped impurity semiconductor films 525 and 526 of thep-channel TFT 563 may be formed by forming a semiconductor film to whichboron (B), which is an impurity imparting p-type, is added.

In this embodiment, the p-channel TFT 563 is used as a pixel TFT of thedual emission display device. Also, the n-channel TFTs 561 and 562 areused as TFTs of a driver circuit that drives the pixel TFT 563. Notethat, it is not required that the pixel TFT is a p-channel TFT, and ann-channel TFT may be used. Further, the driver circuit is not requiredto be a circuit in which a plurality of n-channel TFTs are combined, andit may be a circuit in which an n-channel TFT and a p-channel TFT arecombined complementarily, or a circuit in which a plurality of p-channelTFTs are combined.

Next, as a first interlayer insulating film 541, an insulating filmcontaining hydrogen is formed. Subsequently, the impurity elements addedto the island semiconductor films are activated. This activation of theimpurity elements may be performed by the laser treatment methoddescribed in Embodiment 5.

As the insulating film containing hydrogen, a silicon oxide filmcontaining nitrogen obtained by a PCVD method is used. Alternatively, asilicon nitride film containing oxygen is used. Note that the firstinterlayer insulating film 541 is an insulating film having a lighttransmitting property and containing silicon oxide.

Subsequently, hydrogenation of the island-shaped semiconductor films isperformed by heating at 410° C. for one hour.

Next, a planarization film sewing as a second interlayer insulating film542 is formed. As the planarization film, an inorganic material having alight transmitting property (such as silicon oxide, silicon nitride, orsilicon nitride containing oxygen), a photosensitive ornon-photosensitive organic material (such as polyimide, acrylic,polyamide, polyimide amide, a resist, or benzocyclobutene), or a stackedlayer thereof is used. Further, as another film having a lighttransmitting property used for the planarization film, an insulatingfilm made of a silicon oxide film containing an alkyl group obtained bya coating method, for example an insulating film formed using silicaglass, an alkylsiloxane polymer, an alkylsilsesquioxane polymer, asilsesquioxane hydride polymer, an alkylsilsesquioxane hydride polymer,or the like can be used. As an example of a siloxane-based polymer,PSB-K1 and PSB-K31 which are coating insulating film materials producedby Toray industries Inc. and ZRS-5PH which is a coating insulating filmmaterial produced by Catalysts & Chemicals Industries Co., Ltd. aregiven.

Subsequently, a third interlayer insulating film 543 having a lighttransmitting property is formed. The third interlayer insulating film543 is provided as an etching stopper film for protecting theplanarization film which is the second interlayer insulating film 542,during a later step in which a conductive film serving as a first pixelelectrode 565 is etched. However, if the second interlayer insulatingfilm 542 is to serve as the etching stopper film when the conductivefilm serving as the first pixel electrode 565 is etched, the thirdinterlayer insulating film 543 is not necessary to be provided.

Next, contact holes are formed through the first interlayer insulatingfilm 541, the second interlayer insulating film 542, and the thirdinterlayer insulating film 543 using a new mask. Subsequently, the maskis removed, and after forming a conductive film (a stacked layer film ofa titanium nitride film, an aluminum film, and a titanium nitride film),etching is performed using a different mask (dry etching using a mixedgas of BCl₃ and Cl₂) to form an electrode or wiring 551, an electrode orwiring 552, an electrode or wiring 553, an electrode or wiring 554, anelectrode or wiring 555, and an electrode or wiring 556 (a source wiringor a drain wiring of a TFT, a current supply line, and the like), whichare also collectively referred to as electrodes or wirings 551 to 556(see FIG. 17A). Although an electrode and a wiring are integrally formedin this embodiment, the electrode and the wiring may be separatelyformed and connected electrically. Note that a titanium nitride film isone of materials having favorable adhesiveness with a high heatresistant planarization film.

Subsequently, the first pixel electrode 565 is formed using a new maskso as to have a thickness in the range of 10 nm to 800 nm. As the firstpixel electrode 565, a transparent conductive film having a high workfunction (work function of 4.0 eV or more) such as a conductive filmformed using a target of indium tin oxide containing a Si element, or atarget in which 2 to 20 wt % zinc oxide (ZnO) is mixed with indium oxidebesides a target of indium tin oxide (ITO) can be used.

Next, an insulator 566 (referred to as a partition or a wall) is formedcovering an end portion of the first pixel electrode 565 using a newmask. As the insulator 566, a photosensitive or non-photosensitiveorganic material obtained by a coating method (polyimide, acrylic,polyamide, polyimide amid; a resist, or benzocyclobutene) or an SOG film(for example, a silicon oxide film containing an alkyl group) is usedwith a thickness in a range of 0.8 pin to 1 μm.

Subsequently, a hole injecting layer 571, a hole transporting layer 572,a light emitting layer 573, an electron transporting layer 574, and anelectron injecting layer 575 are formed using an organic compound by anevaporation method or a coating method.

Note that the hole injecting layer 571, the hole transporting layer 572,the light emitting layer 573, the electron transporting layer 574, andthe electron injecting layer 575 are not necessarily formed in thisorder, and can be formed between the first pixel electrode 565 and asecond pixel electrode 576 in the following order: the electroninjecting layer, the electron transporting layer, the light emittinglayer, the hole transporting layer, and the hole injecting layer fromthe first pixel electrode 565 side.

Note that in order to improve reliability of a light emitting element,it is preferable to degas by performing vacuum heating before formingthe hole injecting layer 571. For example, before performing evaporationof the organic compound material becoming the hole injecting layer, itis preferable to perform a heating treatment in a reduced pressureatmosphere or an inert atmosphere at 200° C. to 300° C. to remove gasincluded in the substrate. Note that when the interlayer insulatingfilms and the partition are formed using a silicon oxide film havinghigh heat resistance, a heating treatment at a higher temperature (410°C.) can also be performed.

Subsequently, the hole injecting layer (a first layer containing anorganic compound) 571 is selectively formed over the first pixelelectrode 565 using an evaporation mask by co-evaporating molybdenumoxide (MoOx), 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (α-NPD),and rubrene.

Note that a material having a high hole injecting property such ascopper phthalocyanine (CuPc), vanadium oxide (VOx), ruthenium oxide(RuOx) or tungsten oxide (WOx) can be used besides MoOx. Also, the holeinjecting layer 571 may be formed by a coating method using a highmolecular compound material having a high hole injecting property suchas poly(ethylene dioxythiophene)/poly(styrenesulfonic acid) solution(PEDOT/PSS).

Next, α-NPD is selectively evaporated using an evaporation mask to formthe hole transporting layer (a second layer containing an organiccompound) 572 over the hole injecting layer 571. Note that an organiccompound having a high hole transporting property typified by anaromatic amine compounds such as4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD),4,4′4″-tris(N,N-diphenyl-amino)-triphenylamine (abbreviation: TDATA), or4,4′,4″-tris[N-(3-methyl-phenyl)-N-phenyl-amino]-triphenylamine(abbreviation: MTDATA) can be used besides α-NPD.

Subsequently, the light emitting layer (a third layer containing anorganic compound) 573 is selectively formed. For a full color displaydevice, an evaporation mask is aligned for each light emitting color (R,G, and B) and then selectively evaporated.

As a light emitting layer 573R emitting red light, a material such asAlq₃:DCM or Alq₃:rubrene:BisDCJTM is used. Also, as a light emittinglayer 573G emitting green light, a material such as Alq₃:DMQD(N,N′-dimethylquinacridone) or Alq₃:coumarin 6 is used. Further, as alight emitting layer 573B emitting blue light, a material such as α-NPDor tBu-DNA is used.

Next, Alq₃ (tris(8-quinolinolato)aluminum) is selectively evaporatedusing an evaporation mask to form the electron transporting layer (afourth layer containing an organic compound) 574 over the light emittinglayer 573. Also, besides Alq₃, an organic compound having a highelectron transporting property typified by a metal complex with aquinoline skeleton or a benzoquinoline skeleton such astris(5-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), or the like can be used. In addition, a metal complex having anoxazole-based or thiazole-based ligand such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (abbreviation: Zn(BTZ)₂)can be used. Further, besides the metal complex,2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproin (abbreviation: BCP), or the like also has a high electrontransporting property, and can be used as the electron transportinglayer 574.

Next, 4,4-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) andlithium (Li) are co-evaporated to form the electron injecting layer (afifth layer containing an organic compound) 575 so as to cover theelectron transporting layer 574 and the insulator 566. By using abenzoxazole derivative (BzOs), damage due to the sputtering methodduring formation of the second pixel electrode 576 performed in a laterstep is suppressed. Also, besides BxOs:Li, a material having a highelectron injecting property such as a compound of an alkali metal or analkali earth metal, for example CaF₂, lithium fluoride (LiF), cesiumfluoride (CsF), or the like can be used. In addition, a mixture of Alq₃and magnesium (Mg) can be also used.

Subsequently, the second pixel electrode 576 is formed over the electroninjecting layer 575 so as to have a thickness in a range of 10 nm to 800nm. As the second pixel electrode 576, a conductive film formed using atarget of indium tin oxide containing a Si element, or a target in which2 to 20 wt % zinc oxide (ZnO) is mixed with indium oxide besides atarget of indium tin oxide (ITO) can be used.

In the above manner, alight emitting element is manufactured. A materialand thickness of each of the first pixel electrode 565, the holeinjecting layer 571, the hole transporting layer 572, the light emittinglayer 573, the electron transporting layer 574, the electron injectinglayer 575, and the second pixel electrode 576 which structure the lightemitting element are appropriately selected and set. It is desirablethat the same material be used for the first and second pixelelectrodes, and that they have about the same film thickness, preferablyas thin as about 100 nm.

Also, as described above, the first pixel electrode 565, the electroninjecting layer, the electron transporting layer, the light emittinglayer, the hole transporting layer, the hole injecting layer, and thesecond pixel electrode 576 can be stacked in this order if necessary.

Further, if necessary, a transparent protective layer 577 which preventsentry of moisture is formed covering the light emitting element. As thetransparent protective layer 577, a silicon nitride film, a siliconoxide film, a silicon nitride film containing oxygen, a silicon oxidefilm containing nitrogen, a thin film mainly containing carbon (forexample a diamond like carbon (DLC) film or a carbon nitride (CN) film),or the like obtained by a sputtering method or a CVD method can be used(see FIG. 18).

Further, a second substrate 581 and the substrate 501 are attachedtogether using a sealant containing a gap material for maintaining spacebetween the substrates. The second substrate 581 may also be formed of aglass substrate or a quartz substrate having a light transmittingproperty. Note that in the space between the substrates which are apair, a drying agent as an air gap (inert gas) may be placed, or atransparent sealant (such as an ultraviolet curing or a heat curingepoxy resin) may be filled between the pair of substrates.

Since the first pixel electrode 565 and the second pixel electrode 576of the light emitting element are formed by a light transmittingmaterial, light can be emitted in two directions, in other words fromboth sides, of a single light emitting element.

By having a panel structure as described above, intensities of lightemission from a top surface and a bottom surface can be about the same.

Lastly, optical films (a polarizing plate or a circularly polarizingplate) 582 and 583 are provided over the substrates 501 and 581,respectively, to improve contrast (see FIG. 19).

FIG. 20 shows a cross-sectional view of a light emitting element foreach light emission color (red (R), green (G), blue (B)). A red lightemitting element (R) includes a pixel TFT 563R, a first pixel electrode565R, a hole injecting layer 571R, a hole transporting layer 572R, alight emitting layer 573R, an electron transporting layer 574R, theelectron injecting layer 575, the second pixel electrode 576, and thetransparent protective layer 577.

Also, a green light emitting element (G) includes a pixel TFT 563G, afirst pixel electrode 565G a hole injecting layer 571G, a holetransporting layer 572G a light emitting layer 573G an electrontransporting layer 574G, the electron injecting layer 575, the secondpixel electrode 576, and the transparent protective film 577.

Further, a blue light emitting element (B) includes a pixel TFT 563B, afirst pixel electrode 565B, a hole injecting layer 571B, a holetransporting layer 572B, a light emitting layer 573B, an electrontransporting layer 574B, the electron injecting layer 575, the secondpixel electrode 576, and the transparent protective layer 577.

In this manner, the source region and the drain region of asemiconductor device of this embodiment each have a structure of beingsandwiched from above and below by source or drain electrodes;therefore, contact resistance of the source or drain electrodes and thesemiconductor film can be reduced. Also, since there are two layers ofthe source or drain electrodes, wiring resistance can also be reduced.Further, since the first source or drain electrodes that are formedunder the semiconductor film can be formed to be thin, coverage of thesemiconductor film can be improved to improve TFT characteristics, andreliability of the dual emission display device as a whole can also beimproved.

Further, this embodiment can be freely combined with the embodiment modeand Embodiments 1 to 6, if necessary.

Embodiment 8

As electronic appliances to which the present invention is applied, thefollowing are given: a camera such as a video camera and a digitalcamera, a goggle display, a navigation system, an audio reproducingdevice (such as a car audio component), a computer, a game machine, aportable information terminal (such as a mobile computer, a cellularphone, a portable game machine, and an electronic book), an imagereproducing device provided with a recording medium (specifically, adevice which can reproduce a recording medium such as a digitalversatile disc (DVD) and includes a display capable of displaying theimage), and the like. Specific examples of these electronic appliancesare shown in FIGS. 21A 22D.

FIG. 21A shows a light emitting display device to which a televisionreceiver or the like corresponds. The light emitting display deviceincludes a housing 601, a display portion 603, a speaker portion 604,and the like. The present invention can be applied to the displayportion 603, a control circuit portion, and the like. A polarizing plateor a circularly polarizing plate may be provided in a pixel portion inorder to improve contrast. For example, a ¼λ plate, a ½λ plate and apolarizing plate may be provided in this order over a sealing substrate.Also, an anti-reflection film may be provided over the polarizing plate.By using the present invention, a light emitting display device withhigh reliability can be obtained.

FIG. 21B shows a liquid crystal display or an EL display, and includes ahousing 611, a supporting base 612, a display portion 613, and the like.The present invention can be applied to the display portion 613, acontrol circuit portion, and the like. By using the present invention, aliquid crystal display or an EL display with high reliability can beobtained.

FIG. 21C shows a cellular phone which includes a main body 621, ahousing 622, a display portion 623, an audio input portion 624, an audiooutput portion 625, an operation key 626, an antenna 628, and the like.The present invention can be applied to the display portion 623, acontrol circuit portion, and the like. By using the present invention, acellular phone with high reliability can be obtained.

FIG. 21D shows a personal computer which includes a main body 631, ahousing 632, a display portion 633, a key board 634, an externalconnection port 635, a pointing mouse 636, and the like. The presentinvention can be applied to the display portion 633, a control circuitportion, and the like. By using the present invention, a personalcomputer with high reliability can be obtained

FIG. 22A shows a mobile computer which includes a main body 651, adisplay portion 652, a switch 653, an operation key 654, an infraredport 655, and the like. The present invention can be applied to thedisplay portion 652, a control circuit portion, and the like. By usingthe present invention, a mobile computer with high reliability can beobtained.

FIG. 22B shows a portable game machine which includes a housing 661, adisplay portion 662, a speaker portion 663, an operation key 664, arecording medium insertion portion 665, and the like. The presentinvention can be applied to the display portion 662, a control circuitportion, and the like. By using the present invention, a portable gamemachine with high reliability can be obtained.

FIG. 22C shows a portable image reproducing device provided with arecording medium (specifically, a DVD reproducing device) which includesa main body 671, a housing 672, a display portion A 673, a displayportion B 674, a recording medium reading portion 675 that reads arecording medium such as a DVD, an operation key 676, a speaker portion677, and the like. The display portion A 673 mainly displays imageinformation, and the display portion B 674 mainly displays textualinformation. The present invention can be applied to the display portionA 673, the display portion B 674, a control circuit portion, and thelike. Note that a home-use game machine and the like are also include inthe image reproducing device provided with a recording medium. By usingthe present invention, an image reproducing device with high reliabilitycan be obtained.

FIG. 22D shows a wireless TV of which just a display can be carriedaround. A battery and a signal receiver are incorporated in a housing682, and a display portion 683 and a speaker portion 687 are driven bythe battery. The battery can be repeatedly recharged by a recharger 681.Further the recharger 681 can transmit and receive image signals, andtransmit the image signals to the signal receiver of a display. Thehousing 682 is controlled by an operation key 686. Also, the deviceshown in FIG. 22D can also be called an image sound two-waycommunication device, since it can transmit signals from the housing 682to the recharger 681 by operating the operation key 686. In addition,the device shown in FIG. 22D can also be called a general-purposeremote-control device, since communication control of another electronicappliance is possible by operating the operation key 686 and sendingsignals from the housing 682 to the recharger 681 as well as having theother electronic appliance receive signals that can be transmitted fromthe recharger 681. The present invention can be applied to the displayportion 683, a control circuit portion, and the like. By using thepresent invention, a TV with high reliability can be obtained.

Note that examples shown in this embodiment are mere examples, and thepresent invention is not limited to these uses.

In this manner, a source region and a drain region of a semiconductordevice of this embodiment have a structure of being sandwiched fromabove and below by source or drain electrodes; therefore, contactresistance of the source or drain electrodes and a semiconductor filmcan be reduced. Also, since there are two layers of the source or drainelectrodes, wiring resistance can also be reduced. Further, since thefirst source or drain electrodes that are formed under the semiconductorfilm can be formed to be thin, coverage of the semiconductor film can beimproved to improve TFT characteristics, and reliability of each of theelectronic appliances in which a semiconductor device is incorporatedcan also be improved as a whole.

Further, this embodiment can be applied by being freely combined withthe embodiment mode and Embodiments 1 to 7.

INDUSTRIAL APPLICABILITY

By the present invention, a semiconductor device with reduced contactresistance of source or drain electrodes and a semiconductor film can beobtained. This is because a source region and a drain region of thesemiconductor device of the present invention have a structure of beingsandwiched from above and below by the source or drain electrodes.

Also, since two layers of the source or drain electrodes are formed inthe semiconductor device of the present invention by forming one of themabove an island-shaped semiconductor film and the other under theisland-shaped semiconductor film, wiring resistance can also be reduced.

Further, in the semiconductor device of the present invention, sincefirst source or drain electrodes that are formed under the semiconductorfilm can be formed to be thin, coverage of the semiconductor film can beimproved and TFT characteristics are improved. By the foregoing,reliability of the semiconductor device of the present invention is alsoimproved.

The present application is based on Japanese Priority Application No.2005-300825 filed on Oct. 14, 2005 with the Japan Patent Office, theentire contents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a gate electrode; agate insulating film over the gate electrode; an island-shapedsemiconductor film including a channel forming region over the gateinsulating film; and a first source electrode over the island-shapedsemiconductor film; and a first drain electrode over the island-shapedsemiconductor film, wherein the gate electrode has a taper shape, andwherein the island-shaped semiconductor film comprises indium andoxygen.
 3. The semiconductor device according to claim 2, wherein thefirst source electrode is in direct contact with a side surface of theisland-shaped semiconductor film, and wherein the first drain electrodeis in direct contact with a side surface of the island-shapedsemiconductor film.
 4. The semiconductor device according to claim 2,further comprising: a pixel electrode electrically connected to one ofthe first source electrode and the first drain electrode.
 5. Thesemiconductor device according to claim 2, wherein the island-shapedsemiconductor film has a taper shape.
 6. The semiconductor deviceaccording to claim 2, further comprising: a second source electrodeunder the first source electrode with the island-shaped semiconductorfilm located therebetween, and a second drain electrode under the firstdrain electrode with the island-shaped semiconductor film locatedtherebetween.
 7. The semiconductor device according to claim 2, whereinthe island-shaped semiconductor film comprises a source region betweenthe channel forming region and the first source electrode, and whereinthe island-shaped semiconductor film comprises a drain region betweenthe channel forming region and the first drain electrode.
 8. Thesemiconductor device according to claim 2, wherein the island-shapedsemiconductor film comprises a first region capable of reducing acontact resistance between the island-shaped semiconductor film and thefirst source electrode, and wherein the island-shaped semiconductor filmcomprises a second region capable of reducing a contact resistancebetween the island-shaped semiconductor film and the first drainelectrode.
 9. The semiconductor device according to claim 2, wherein theisland-shaped semiconductor film is an indium oxide film.